This invention relates to improvements in a sound-absorbing duct structure intended to sound absorption and insulation in automotive vehicles, architectural structures and the like, and more particularly to the sound-absorbing duct structure which exhibits excellent sound-absorbing effects throughout whole frequency ranges from low to high frequency ranges.
Hitherto much attention has been paid on noise generated in intake and exhaust systems for gaseous fluid, in which sources of the noise are, for example, noise generated by a blower, air stream noise of a blower, a high pressure valve and nozzle, intake and exhaust noise of an internal combustion engine and a gas generating machine, and burning noise of an oil burner or a gas burner. In order to reduce such noise, mufflers (silencers) and the like have been used in the intake and exhaust systems. Additionally, a sound-absorbing material such as glass wool has been used to reduce air stream noise in a high frequency range which noise is generated at a certain flow speed of the fluid flowing in a duct.
Reduction of such noise has been hitherto accomplished by minimizing a flow resistance of the fluid relative to the surface of the duct, by attenuating an acoustic energy serving as a noise source as much as possible, or by installing the sound-absorbing material, the mufflers and the like. Structures such as the mufflers are effective for noise in a particular frequency range but are not effective for noise in whole frequency ranges. The sound-absorbing materials are effective only for noise in a relatively high frequency range.
Air intake noise in an internal combustion engine is originated from pulsation of intake air of the engine and mainly in a low frequency range lower than 500 Hz. A sound-absorbing structure such as a resonator or a side branch structure is installed as a main measure in order to reduce such air intake noise. However, this sound-absorbing structure can exhibit noise reduction effect only for noise in a particular frequency range. Accordingly, installation of a plurality of such sound-absorbing structures is required for the purpose of reducing noise in a wide frequency range. It will be understood that such installation of the plurality of the sound-absorbing structures are difficult in case that a space for the sound-absorbing structures is limited or relatively small.
In addition, the following propositions on sound-absorbing duct structure have been hitherto made in order to reduce the above noise: A plurality of small holes are formed in an air intake pipe between a carburetor and an air cleaner, in which a sound-absorbing material is disposed around the air intake pipe, as disclosed in Japanese Patent Publication No. 53-148617 and Japanese Utility Model Provisional Publication No. 55-167562. A partition wall is provided to separate the side of an internal combustion engine and the side of an air cleaner element, in which the partition wall is formed with an orifice, as disclosed in Japanese Patent Provisional Publication No. 64-53055. A bypass tube is used in an air cleaner casing and a duct for the purpose of reducing noise, as disclosed in Japanese Patent Provisional Publication No. 5-18329. A particular resonance duct is connected to an air cleaner casing thereby to attenuate resonance in a particular frequency range, as disclosed in Japanese Patent Provisional Publication No. 5-18330. A sound-absorbing material is installed to the vicinity of an open end section of a duct structure; however, this is hardly effective for noise in a low frequency range.
Concerning noise in an intake system of an automotive vehicle, noise in a low frequency range lower than 500 Hz is basically problematic though the frequency range of noise changes in accordance with engine speed. Accordingly, it is desirable to obtain a sound-absorbing structure effective for reducing noise in whole frequency ranges including the above low frequency range. Additionally, it is also desirable to so arrange the sound-absorbing structure as to have a high performance and a compact configuration.
In view of the above, an object of the present invention is to provide an improved sound-absorbing duct structure for effectively overcoming drawbacks encountered in conventional sound-absorbing duct structures.
Another object of the present invention is to provide an improved sound-absorbing duct structure which is high in noise reducing effect particularly in a relatively low frequency range, while it can take a compact configuration.
A further object of the present invention is to provide an improved sound-absorbing duct structure which can effectively reduce noise in a wide frequency range, particularly a low frequency range, generated in a duct of intake and exhaust systems of an internal combustion engine, without providing any complicated devices.
An aspect of the present invention resides in a sound-absorbing duct structure comprising a sound-absorbing section arranged in communication with a gas passage. The sound-absorbing section includes an piezoelectric material exhibiting a piezoelectric effect, and an electrically conductive material in contact with the piezoelectric material.
Another aspect of the present invention resides in a sound-absorbing duct structure comprising a duct defining thereinside a gas passage, a part of the duct being formed with a plurality of small holes. An expanded duct section is formed around the part of the duct. The expanded duct section defines therein an inside space which is in communication with the gas passage through the small holes, the inside space having a cross-sectional area larger than that of the gas passage. A sound-absorbing section is disposed in at least a part of the inside space of the expanded duct section. The sound absorbing section includes a piezoelectric material exhibiting a piezoelectric effect, and an electrically conductive material in contact with the piezoelectric material.
A third aspect of the present invention resides in a sound-absorbing duct structure for an intake system of an automotive internal combustion engine. The sound-absorbing duct structure comprises a duct defining thereinside a gas passage, a part of the duct being formed with a plurality of small holes. The duct is connected to an air intake duct of an air cleaner so that the gas passage forms part of an air intake passage through which intake air is supplied to the engine. An expanded duct section is formed around the part of the duct and defines therein an inside space which is in communication with the gas passage through the small holes, the inside space having a cross-sectional area larger than that of the gas passage. A sound-absorbing section is disposed in at least a part of the inside space of the expanded duct section, the sound absorbing section including an piezoelectric material exhibiting a piezoelectric effect, and an electrically conductive material in contact with the piezoelectric material.
A fourth aspect of the present invention resides in a method of absorbing sound in a gas passage. The method comprises
(a) disposing a sound-absorbing section in communication with the gas passage, the sound-absorbing section including an piezoelectric material exhibiting a piezoelectric effect, and an electrically conductive material in contact with the piezoelectric material; and
(b) causing a gas to flow through the gas passage so as to allow a sound pressure due to flow of the gas is absorbed in the sound-absorbing section.